The present invention relates to friction clutches in general, particularly to pull-type friction clutches wherein the radially inwardly extending tongues of the diaphragm spring must be pulled in order to disengage one or more clutch discs from one or more pressure plates.
Diaphragm springs for use in friction clutches are normally provided with the aforementioned tongues which extend substantially radially inwardly from the energy-storing annular main section of the spring and which alternate with substantially radially extending slots communicating with a centrally located opening. It is further known to provide such types of diaphragm springs with prongs which extend radially outwardly of the main section and can be used as a means for facilitating attachment of the diaphragm spring to the flywheel on the crankshaft of the engine in an automotive vehicle. The diaphragm spring bears against the pressure plate in the region of its main section to thereby urge the pressure plate against the clutch disc when the clutch is engaged. A similar friction clutch is disclosed, for example, in German Pat. No. 19 43 336 wherein the diaphragm spring has radially outwardly extending slotted prongs which serve as a means for facilitating prestressing of the spring by bolting the prongs to the flywheel to thereby urge the main section of the spring against protuberances on the adjacent pressure plate. The prongs are designed to roll along suitable surfaces and such rolling takes place when the conicity of the diaphragm spring (inclusive of the main section and prongs) is changed for the purpose of engaging or disengaging the clutch. An advantage of such friction clutches is their simplicity. However, the wear upon the prongs and upon the surface which are in rolling contact with the prongs is rather pronounced with the result that the extent of deformation of the diaphragm spring (for the purpose of engaging or disengaging the clutch) must be increased proportionally with the increasing wear upon the prongs.
German Offenlegungsschrift No. 30 17 563 discloses a modified friction clutch wherein the prongs of the diaphragm spring are fixedly secured to the flywheel without any freedom of rolling movement. When the clutch is to be disengaged, the radially inwardly extending tongues of the diaphragm spring are acted upon in such a way that the conicity of the spring is changed whereby the prongs are flexed with reference to the adjacent portions of the flywheel. This entails the development of frequently recurring stresses which shorten the useful life of the diaphragm spring. The design of the prongs is not best suited to enable the prongs to stand such stresses. Additional pronounced stresses develop in the regions where the prongs merge into the main section of the diaphragm spring, and such stresses are superimposed upon stresses which normally develop in the main section. Such superimposition of stresses is attributable to the fact that the diaphragm spring which is disclosed in the just mentioned German publication does not allow for flexing of the radially innermost portions of prongs with reference to the annular main section and/or vice versa. Thus, the main section undergoes deformation in the regions of merger of the prongs whenever the diaphragm spring is caused to change its conicity for the purpose of engaging or disengaging the clutch. The situation is aggravated because the prongs are relatively short, as considered in the radial direction of the diaphragm spring; this contributes to the development of very pronounced stresses which are superimposed upon the normally developing stresses in the main section of the spring. The result is a substantial shortening of the useful life of the diaphragm spring and, as a rule, of the friction clutch which embodies the spring.